The present invention relates to improvements in the field of high-speed dynamic printing. More particularly, the invention is concerned with an improved electrocoagulation printing method and apparatus wherein dots of electrocoagulated colloid representative of an image are transferred onto a substrate, such as paper.
Applicant has already described in his U.S. Pat. No. 4,661,222 of Apr. 28, 1987 an electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a moving endless belt onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate to cause transfer of the coloring agent used for coloring the colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the coloring of the colloid is effected either before or after the coagulation thereof depending on whether the coloring agent used is a pigment or a dye. Where the coloring agent is a pigment, the coloring of the colloid is effected prior to coagulation and the dots of colored, coagulated colloid obtained upon coagulation of the colored colloid must be treated with a colloid softening agent so as to maintain the colored, coagulated colloid in a softened state for enabling the pigment to be subsequently transferred onto the substrate. In the case where the coloring agent is a dye, the coloring of the colloid is effected after coagulation by applying to the dots of coagulated colloid a liquid coloring medium containing the dye, thereby obtaining the desired dots of colored, coagulated colloid. In this case, however, the substrate must be coated with a wetting agent which is a solvent of the dye for enabling the dye to be transferred onto the substrate and, depending on the type of substrate used, further treatment may be necessary. For instance, if gelatinized paper is used, the wetting agent must also act as a gelatin softening agent for conditioning the gelatinized paper to receive the dye. If bond paper or synthetic resin-coated or kaolin-coated paper or the like is used, the coloring medium must further contain a colloid softening agent so as to maintain the colored, coagulated colloid in a softened state and thus allow transfer of the dye onto such type of substrate.
Since the coloring agent is transferred onto the substrate, the period of contact between the dots of colored, coagulated colloid and the substrate must be sufficiently long to enable all of the coloring agent to be transferred onto the substrate. Such a transfer generally takes about 1 second. Where the coloring agent used is a dye, sufficient time must also be alloted for the dots of coagulated colloid to absorb the dye. Thus, it may take up to 3 seconds from the time the dye is applied to the dots of coagulated colloid till complete transfer of the dye onto the substrate. Use of a substrate coated with a wetting agent such as an alcohol wetted paper for enabling the dye to be transferred also causes difficulties in superimposing with precision images of different colors.
Moreover, since the negative electrodes are generally energized more than once in the reproduction of an image, these become polarized resulting in secondary electrolytic reactions causing the generation of gas bubbles which remain trapped at the interface of the negative electrodes and thus adversely affect the image reproduction. Edge corrosion of the negative electrodes has also been observed upon repeated electrocoagulation, necessitating replacement of the electrodes every 1000 printed copies.
The problem of undesirable gas generation at the negative electrodes has been solved by Applicant in his U.S. Pat. No. 4,680,097 of July 14, 1987. According to the teaching of this patent, undesirable gas generation and accumulation at the negative electrodes is prevented by coating the positive electrode with an olefinic substance prior to electrically energizing the electrodes such that upon electrical energization gas generated as a result of electrolysis is consumed by reaction with the olefinic substance; such a reaction must however be carried out in the presence of a metal oxide which acts as a catalyst and which is either already present as a surface layer on the positive electrode or is admixed with the olefinic substance. Coating of the positive electrode with an olefinic substance has also been found to weaken the adherence of the dots of coagulated colloid to the positive electrode, thereby permitting transfer of the coagulated colloid onto the substrate upon contact therewith. However, there still remained the problem of edge corrosion of the negative electrodes.
When applying the technology of the above patent to high-speed electrocoagulation printing and using a positive electrode in the form of a revolving cylinder instead of a moving endless belt to increase the speed of transfer of the coagulated colloid, Applicant was faced with still another problem: corrosion of the positive electrode creating undesirable marking of the electrode surface and adversely affecting the image reproduction.